1. Field of the Invention
Embodiments of the invention relate generally to hard disk drives. More particularly, embodiments of the invention relate to methods of measuring track width variation in hard disk drives, and recording media adapted to store computer programs used by the methods.
A claim of priority is made to Korean Patent Application No. 10-2006-0009606, filed on Feb. 1, 2006, the disclosure of which is hereby incorporated by reference in its entirety.
2. Description of Related Art
Hard disk drives (HDDs) are commonly used to provide data storage for computer systems. Hard disk drives are characterized by providing vast amounts of data storage and efficient access to stored data. In general, hard disk drives are formed by a combination of electronic and mechanical parts used to record data on a magnetic storage medium by converting digital signals into magnetic fields in order to magnetize portions of the magnetic storage medium. The electronic and mechanical parts are also used to reproduce data stored on the magnetic storage medium by converting magnetic fields produced by magnetized portions of the magnetic storage medium into digital signals.
In most hard disk drives, data is stored in concentric circular tracks on the magnetic storage medium. Accordingly, in order to accurately read and record data on these tracks, a read/write head of the hard disk drive must be properly positioned with respect to the tracks. In modern “embedded servo” hard disk drives, this positioning is accomplished by programming “servo data,” or “servo patterns” at various angular positions of each concentric track using a servo writer and then using the servo data to sense the head's position as it moves along the tracks. The servo data allows the HDD to sense the head's position because the servo data produces different output signals in the head depending on the head's position in relation to the center of each track. The servo data is typically programmed on the HDD when the HDD is manufactured and then remains the same throughout the life of the HDD. Accordingly, it is important that the servo data is accurately placed at the time of a hard disk drive's manufacture.
One problem that can occur in the manufacture of hard disk drives is that imprecise programming of the servo data can cause the tracks to have undesirable width variation, both within each track and between tracks. In order to form each track with a consistent width, the servo writer must program the servo data with a high degree of precision. If the servo writer is not precisely positioned when the servo data is programmed on a HDD, the tracks of the HDD will have different, non-uniform widths. Where this happens, the head may not be able to achieve accurate positioning for reading and writing data on the HDD.
For example, FIG. 1 is a graph illustrating the positioning of data written on a HDD. Referring to FIG. 1, portions of three different tracks are illustrated by a rectangular grid. In each grid, white blocks indicate portions of the track that have been written. As can be seen in the grids, upper portions of each track are written and lower portions of each track are not written. In other words, the data on of these tracks is written slightly off the center of each track. One reason for this off-center writing of these tracks is that the tracks do not have constant widths, as defined by the servo data.
There are a variety of methods for recording servo data in hard disk drives. However, it is difficult to form ideal servo patterns due to inherent limits of conventional servo writer technology. Accordingly, to address the problems inherent in the servo writer technology, track width variation is typically monitored when the servo writer writes the servo data by explicitly measuring track width variation. In other words, after the servo writer writes the servo data, an additional step is performed to detect the width of resulting data tracks. In addition, further steps may also be performed to verify the quality of servo patterns themselves.
In a conventional method for measuring a track width, a head is moved to a predetermined distance from a 0% off track. The term “off track” here refers to a relative displacement of the head from a track's center. A percentage of off track is measured in a minus (−) direction on one side of the track center, and a plus (+) direction on an opposite side of the track center. Once the head is moved to the predetermined distance from the 0% off track, the head measures the track width in relation to “servo bursts”, or burst signals, which form part of the servo data.
The above method uses two different limits to evaluate the severeness of track width variation. The first limit relates to an overall width difference between a track and neighboring tracks. The second limit relates to variation of the widths of sides of each track with respect to the center of the track.
As an example, FIG. 2 is a graph illustrating measurements of a track's width taken using a conventional method. In FIG. 2, there are two curves corresponding to the amplitudes of signals produced in the head of a hard disk drive in relation to burst signals when the head is moved along the track on opposite sides of the track's center. As seen in FIG. 2, there is significant variance within each of the measured signals, and the there is also significant variance in the distance between the signals. The variance within each signal can be interpreted as variation in the width of the track, and the variation in the distance between the signals can be interpreted as variation in the widths of neighboring tracks in the hard disk drive. However, much of the variation can also be attributed to noise in the measurement process, e.g., in the form of unknown variations in the amplitudes of burst signals used to generate the signals.
FIG. 3 is another graph illustrating measurements of a track's width taken using a conventional method. Like FIG. 2, FIG. 3 contains two curves corresponding to the amplitudes of signals produced in relation to burst signals when a head is moved along the track on opposite sides of the track's center. Like the curves in FIG. 2, the curves in FIG. 3 also exhibit significant variation, indicating possible variation in track width. However, the curves in FIG. 3 have a more clear separation than the curves in FIG. 2, potentially indicating the width of the curves and the widths of neighboring curves.
In conventional methods for measuring track widths in a hard disk drive, track width variation between neighboring tracks is measured under an assumption that each servo burst among the servo data in the tracks has substantially the same electromagnetic properties, e.g., the same amplitude. In addition, the track width variation is also measured under an assumption that a center point of each track and therefore deviation of written data from the center point, can be identified with some degree of accuracy.
Unfortunately, however, the amplitude of the servo bursts can vary, making the measurements of the track widths and the track center points somewhat unreliable. Accordingly, the range of allowable measured track width variation must be adjusted to account for the uncertainty in the amplitudes of the servo bursts. Moreover, the allowable measured track width variation must also be adjusted when measuring the track widths of hard disk drives having different burst signal amplitudes or different number of tracks per inch (TPI) due to differences in the accuracy of corresponding servo writers.